DOVER, Fla. — The F.B.I. agent who spurred the investigation that led to the resignation of David H. Petraeus as C.I.A. director is a “hard-charging” veteran who helped investigate the foiled millennium terrorist plot in 1999, colleagues said on Wednesday.

The agent, Frederick W. Humphries II, 47, is also described by former colleagues as relentless in his pursuit of what he sees as wrongdoing, which appears to describe his role in the F.B.I. investigation involving Mr. Petraeus. Suspecting that the case involved serious security issues and was being stalled, possibly for political reasons — a suspicion his superiors say was unjustified — he took his concerns to Congressional Republicans.

“Fred is a passionate kind of guy,” one former colleague said. “He’s kind of an obsessive type. If he locked his teeth onto something, he’d be a bulldog.”

The question of how and why the F.B.I. opened the investigation that has had such momentous consequences has been central from the moment Mr. Petraeus stepped down Friday. The emerging portrait of the agent who initiated the inquiry is another step toward an answer.

Mr. Humphries, who was identified on Wednesday by law enforcement colleagues, took the initial complaint from Jill Kelley, a Tampa woman active in local military circles and a personal friend, about anonymous e-mails that accused her of inappropriately flirtatious behavior toward Mr. Petraeus.

The subsequent cyberstalking investigation uncovered an extramarital affair between Mr. Petraeus and Paula Broadwell, his biographer, who agents determined had sent the anonymous e-mails. It also ensnared Gen. John R. Allen, the commander of NATO forces in Afghanistan, after F.B.I. agents discovered what a law enforcement official said on Wednesday were sexually explicit e-mail exchanges between him and Ms. Kelley.

A spokesman for Ms. Kelley provided her version of events in two conference calls with reporters on Wednesday. Ms. Kelley’s concern when she took the e-mails to Mr. Humphries was that she feared the sender was “stalking” Mr. Petraeus and General Allen, said the spokesman, who asked not to be identified.

“She asks the agent, ‘What do you make of this?’ ” the spokesman said. “The agent said: ‘This is serious. They seem to know the comings and goings of a couple of generals.’ ”

General Allen himself had received a similar anonymous e-mail message, sent by someone identified as “kelleypatrol,” advising him to stay away from Ms. Kelley. The general forwarded it to Ms. Kelley, and they discussed a concern that someone was cyberstalking them.

On Tuesday, Defense Secretary Leon E. Panetta said he had asked the Senate to postpone a confirmation hearing for General Allen’s next assignment while the department’s inspector general reviewed his e-mail correspondence with Ms. Kelley, which was discovered by F.B.I. agents investigating her initial complaint.

Photo

Frederick W. Humphries II at the F.B.I. office in Seattle in 1999.Credit
Steve Ringman/The Seattle Times

Pentagon officials said the review covered more than 10,000 pages of documents that included “inappropriate” messages. But associates of General Allen have said that the two exchanged about a dozen e-mails a week since meeting two years ago and that his messages were affectionate but platonic.

A law enforcement official, speaking on the condition of anonymity, disputed that assertion on Wednesday, saying some messages were clearly sexual. Investigators were confident “the nature of the content warranted passing them on” to the inspector general, the official said.

In a statement on Wednesday, General Allen’s military counsel said he intended to cooperate fully with the inspector general’s investigation. “To the extent there are questions about certain communications by General Allen, he shares in the desire to resolve those questions as completely and quickly as possible,” said the statement from Col. John G. Baker, the chief defense counsel of the Marine Corps.

The F.B.I. director, Robert S. Mueller III, and the deputy director, Sean Joyce, briefed leaders of the Senate and House intelligence committees about the investigation. Mr. Petraeus is expected to speak to the panel behind closed doors on Friday about the attack on the American Mission in Benghazi, Libya. The events leading to his resignation are certain to come up.

A Pentagon official said the security clearance of Ms. Broadwell, a West Point graduate and officer in the Army Reserve, had been suspended pending the outcome of the F.B.I. investigation. F.B.I. agents on Monday night carried boxes of documents and a computer out of the house she shares with her husband and two sons. The law enforcement official said that Ms. Broadwell had cooperated with investigators in their effort to remove all classified material, which by law cannot be kept in an insecure facility.

The officials said Ms. Kelley was no longer permitted to enter MacDill Air Force Base in Tampa “with a wave,” as she has for years as a regular volunteer and visitor to ranking officers there. Now she has to get approval and sign in at the visitor’s gate, the official said.

Ms. Kelley, whose house has been besieged by reporters and television crews, has called 911 several times to complain about snooping reporters, according to tapes and transcripts of the calls posted on the Web. In at least one call, she asked for “diplomatic protection,” saying she is an “honorary consul general,” a designation she reportedly received from South Korean diplomats.

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By all accounts, Mr. Humphries doggedly pursued Ms. Kelley’s cyberstalking complaint. Though he was not assigned to the case, he was admonished by supervisors who thought he was trying to improperly insert himself into the investigation.

In late October, fearing that the case was being stalled for political reasons, Mr. Humphries contacted Representative Dave Reichert, a Republican from Washington State, where the F.B.I. agent had worked previously, to inform him of the case. Mr. Reichert put him in touch with the House majority leader, Eric Cantor, who passed the message to Mr. Mueller.

Lawrence Berger, the general counsel for the Federal Law Enforcement Officers Association, who spoke with Mr. Humphries, said Mr. Humphries only received the information from Ms. Kelley and never played a role in the investigation.

Mr. Berger said Mr. Humphries and his wife had been “social friends with Ms. Kelley and her husband prior to the day she referred the matter to him.”

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Frederick W. Humphries II helped start the inquiry that led to the resignation of David H. Petraeus, center, as C.I.A. director and ensnared the commander of NATO forces in Afghanistan, Gen. John R. Allen, left, shown with Leon E. Panetta in 2011.Credit
Pool photo by Paul Richards

“They always socialized and corresponded,” he said.

Mr. Berger took issue with news media reports that said his client had sent shirtless pictures of himself to Ms. Kelley.

“That picture was sent years before Ms. Kelley contacted him about this, and it was sent as part of a larger context of what I would call social relations in which the families would exchange numerous photos of each other,” Mr. Berger said.

The photo was sent as a joke, he said, and was of Mr. Humphries “posing with a couple of dummies.” Mr. Berger added that it was not sexual in nature.

Two former law enforcement colleagues said Mr. Humphries was a solid agent with experience in counterterrorism. He has conservative political views and a reputation for being aggressive, they said.

Colleagues and news reports described the role of Mr. Humphries, who in 1999 was in his third year at the F.B.I., in building the case against Ahmed Ressam, who was detained as he tried to enter the United States from Canada with a plan to set off a bomb at Los Angeles International Airport.

In May 2010, after he had moved to the Tampa field office, Mr. Humphries fatally shot a knife-wielding man near a gate of MacDill Air Force base. A state prosecutor declined to prosecute the case, and the Justice Department’s civil rights division and an internal F.B.I. review board each also found that the use of force had been justified, according to bureau records.

A large American flag was flying on Wednesday in front of Mr. Humphries’s house in Dover, a half-hour drive from Tampa. A man standing in the driveway who appeared to be Mr. Humphries, approached by a reporter seeking comment, said his first name was not Fred. The man then walked into the house, closed the front door and did not respond to the doorbell.

In regard to his client’s speaking with Mr. Cantor, Mr. Berger declined to address the issue, saying only that his client “had followed F.B.I. protocols.”

“No one tries to become a whistle-blower,” he said. “Consistent with F.B.I. policy, he referred it to the proper component.”

A law enforcement official said disclosing a confidential investigation even to Congress members could sometimes violate F.B.I. rules. The official said that Mr. Humphries’s conduct was under review and that he had not been punished in any way.

On Wednesday, Mr. Cantor said he had no intention of “politicizing” the tip from Mr. Humphries, whom he did not name. “The information that was sent to me sounded as if there was a potential for a national security vulnerability,” Mr. Cantor said.

A version of this article appears in print on November 15, 2012, on Page A1 of the New York edition with the headline: F.B.I. Agent in Petraeus Case Is Called a Relentless Veteran. Order Reprints|Today's Paper|Subscribe